Capacitive Deionization and Disinfection of Water Using Graphene Oxide Dendrimer-Ag Electrodes

Introduction

With population growth, industrial development, and increased pollution of freshwater sources, access to adequate and safe water has become a serious crisis in some countries.
Most of currently in use water disinfectants are not fully effective in destroying pathogenic microorganisms in drinking water, or they may produce side products that are harmful to human health. Also, some of these disinfectants are toxic.
Today, finding special methods with high efficiency, appropriate cost, and impact on the wide range of pathogenic bacteria, has been highlighted because of resistant microbial contamination of conventional treatments, as well as other disadvantages of existing disinfection methods.
At present, nanoscale with a multi-purpose antibacterial potential has created hopes for solving the problem of drug-resistant bacteria.
Silver is one of the most effective antibiotics known in human history and can destroy more than 650 different bacteria in few minutes, and have been used in antimicrobial applications since the 19th century.
Despite the good antibacterial property of silver, silver nanoparticles due to its surface properties and photocatalytic properties can facilitate oxidative damage to nearby cells, and also have remarkable bio-toxic activity due to the sustained release of Ag +.
A solution to prevent the release of silver nanoparticles and silver ions into the water was found to utilize the antimicrobial properties of nano silver.
A new method of antimicrobial agents is to place an antimicrobial agent on the surface of carriers, such as graphene oxide, activated carbon and composite formation.
Graphene oxide, has been a good selection for composite materials. GO contains two-dimensional materials composed of carbon atoms in a crystal honeycomb grid.). GO, and composites of GO using chemical and physical mechanisms have been reported to have antibacterial properties.
Also, researchers have shown that due to the synergistic effect of silver nanoparticles and graphene oxide, the combination of these two substances has a better antibacterial effect.
Despite the antimicrobial properties of GO-Ag nanocomposite, due to the nanoscale thickness of these materials, GO-Ag nanocomposite is stable in water, and it is not possible to remove it from drinking water with conventional methods. Therefore, for the use of GO-Ag nanocomposites to disinfect drinking water, a method should be used that can stabilize these substances and prevent them from releasing and suspending in water.
Capacitive deionization (CDI) technology is a method for removing ions from brackish water with active carbon electrodes. The overall efficiency of CDI system depends on the surface properties of the electrode, such as the specific surface area, the distribution and porosity size, the absorption characteristics and the main chemical groups at the electrode surface. The good results of a capacitive deionization system in water desalination, have the potential to use this system for the removal of organic matter, microbial agents, and other water contaminants.
The capacitive deionization process is a continuous flow process, which is performed by ion absorption and ion desorption alternating cycle, to remove ions from water and regeneration stage, respectively which is applied to a direct voltage.

Due to the low energy consumption (2 V), good regeneration ability, and Non-use of chemicals at different stages of desalination, the CDI system has been considered
The CDI technology with enhanced and improved electrodes can be a good solution for disinfecting and desalination water.
In this research, a new coating of graphene oxide dendrimer silver on the activated carbon electrodes was introduced for high desalination and disinfection capacity of water by a capacitive deionization-disinfection system.

2. Methods and method
 Materials
Graphene oxide (GO), Activated carbon (AC), hydrochloric acid (HCL), Sodium hydroxide (NaOH), Epichlorohydrin (C3H5ClO), Sodium borohydride (NaBH4), Sodium Sulfide (Na2S), sodium acetate (CH3COONa), silver nitrate (AgNO3) were supplied from Merck Co, Germany.

 Method
 Dendrimer grafting on the graphene oxide (GO-D)
 Preparation of Silver nanoparticle
 Nanocomposite CDID Electrode Fabrication
 Measurements and characterizations
- FTIR
- TGA
- FE-SEM/EDS
 Initial solution preparation
- NaCl solutions with different TDS (1000 to 20000 mg/ L)
- initial MPN of coliform (1000, 5000, 10000 and 100000 MPN)

3. Results and discussion
3.1. Effect of initial TDS and contact time
Nacl solution at concentrations of 1000, 1500, 5000 and 20,000 mg / L was tested for a 5 to 90 minutes’ contact time to determine the deionization ability of the water. to compare the salt solution and the actual sample of water, Caspian Sea water was tested with a TDS of 20000 mg/l. Experimental results show that the system has high capability of deionizing water.
Also to determine the simultaneous deionization and disinfection capability, NaCl solution with concentrations of 500, 1000, 1500 mg/l and 5000 mg/l was tested with 10000 MPN of initial coliform and 15 to 60 minutes’ contact time to determine the impact of TDS on coliform removal
Experimental results showed that the system had ability of removing 80% of water salinity simultaneously with water disinfection.


3.2. Effect of initial coliform
One of the parameters in the evaluation of the disinfection activity has been initial coliform. For this purpose, various coliforms (500, 1000, 10000 and 100000 MPN) were tested in NaCl solution 500 mg/,30 and 60 minutes’ contact time.
Killing rate of coliform in the initial 500 and 1000 MPN was 99.9% at 30 minutes. This retention time was similar to the time required for disinfection in the conventional chlorination system in water treatment plant but without the use of chemicals for disinfection and remaining by product in disinfected water.
It also had the ability to remove 99.9% of 10000 MPN in 60 minutes.

4. Conclusion
GO-D-AG is a new nanocomposite, synthesized from graphene oxide with a new generation of dendrimers, with silver nanoparticles on branches, to make new electrodes for CDID system. A thin layer of GO-D-Ag nanocomposite will be located on the surface of carbon electrodes.
Due to the covalent bonding between dendrimer and graphene oxide and the strong complexation between sulfur and silver nanoparticles in the branches, this system is a safe solution for the use of silver nanoparticles in water disinfection.
The amount of nanoparticles used in the electrodes had a great effect on the disinfection property of the capacitive deionization system.
Contact time was also an effective factor in reducing coliform from water. Experiments were evaluated for 15 to 120 minutes on contaminated water with coliform content of 1000 to 100000 MPN.
This electrode had a disinfection effect of at least 99.9% (4 Log Reduction of coliform) and 90% (5 Log Reduction of coliform) with 60-minute contact time and completely removed 100000 MPN of coliform in 120-minute contact time. Experiments have shown that these electrodes were capable of disinfecting water at salinity levels up to 5000 mg/l, whereas in the previous research, only disinfection at 200 mg / L was performed by CDI system . Disinfection by this CDID system with the new generation of the electrode can be an excellent alternative to traditional disinfection processes because it had low energy consumption (2 V uses for disinfection), quick and useful regeneration, and disinfection without any byproduct.